Has this just started after a lift? I would say tire out of balance if it goes away above 65 , or at least diminishes. But you say it is fine with front drive shaft off, so that brings me back to my first question, Here is the problem with the drive shaft you have, It is a prop shaft or CV style shaft. the Length is factory set. It does not allow for the different lenght you now need due to the lift like say a U-joint with the slip type Yoke. I am not sure if it will work with 3" / 4" and I know we all start to have issues with that prop shaft. It is a tough one. so first try to rotate the tires front to rear, and defiantly check your tie rod ends as mention. at least to be safe. Then it is back to the front drive shaft being the culprit. find out what was done to it. and was it good right after the rebuild or replacement of you new one?.